Heat-shrinkable plastic films are widely used in applications such as shrinkage packaging, shrink labels, and the like due to their characteristic property of shrinking by heat. Among them, polyvinyl chloride, polystyrene, polyester and other drawn films are used for packaging various containers made of materials such as polyethylene terephthalate (PET), polyethylene, glass, and as labels and cap seals or for the purpose of collective packaging.
However, polyvinyl chloride films have problems that they are low in heat resistance and generate hydrogen chloride gas and dioxins during incineration. In addition, the heat-shrinkable polyvinyl chloride resin film is used as shrink labels for PET containers or the like there is a problem that the PVC labels would be separated from the containers if the containers are to be recycled and reused.
Alternatively, polystyrene films are reasonably more favorable in label appearance after shrinkage but poorer in solvent resistance, and thus demand inks that contain special ingredients for printing. Additionally, polystyrene resins demand a higher temperature for incineration and have a problem of generating black smoke and foreign odor in greater amounts during incineration.
Having no such problems, polyester films are highly expected as shrink labels replacing the polyvinyl chloride and polystyrene films, and the amount of their consumption is gradually increasing in parallel with the expansion in the amount of PET containers used.
After production, these heat-shrinkable films are once wound in the form of roll; and then sent in such a form to a printing process wherein various drawings are printed thereon. If desired, these heat-shrinkable films are slit to a size suitable for use as labels for wrapping end-products after printing. These heat-shrinkable films are then converted into a tubular form by superimposing and sealing both ends of the films by means of, for example, solvent bonding and the like, and cut into the form of labels, bags or the like. The labels and bags thus obtained are loaded on containers, and then tightly bound to the containers by heat shrinkage, as the wrapped containers are passed on a belt conveyer or the like through a shrinkage tunnel (steam tunnel) wherein steam is applied for the heat shrinkage or in a shrinkage tunnel (hot-air tunnel) wherein heated air is applied for the heat shrinkage.
In the progressive move toward restriction of the use of colored PET bottles from the viewpoint of material recycling, there exists an increasing demand for larger labels of heat-shrinkable polyester films that can cover most of the sidewall of the bottles (so-called full labels) for the purpose of replacing the colored bottles. However, the sidewall shapes of the PET bottles vary significantly and accordingly the diameters of a PET bottle at particular heights vary significantly. Therefore, if a single label is used for wrapping a bottle, the extent of desirable label shrinkage varies depending on the specific position of the label wrapped around the bottle. Therefore, there exists a need for a heat-shrinkable polyester film having a shrinkage property superior to the conventional films and a potential of exhibiting an excellent label shrinkage property even when used for wrapping the bottles having an irregular sidewall shape.
For example in the production of PET-bottled drinks, loading and shrinkage of such labels is carried out more frequently in the drink filling line, for the purpose of improving productivity. Generally such filling lines are operated at high speed, thus demanding high-speed loading and shrinkage of the labels, i.e., shrinkage in a shorter period of time. As a result, in demand are heat-shrinkable polyester films that have the physical properties sufficient large for enduring the high-speed loading and the shrinkage property allowing higher shrinkage in a shortened period of time.
In addition, recently, labels for use in wrapping PET bottles and other various containers are required to have an additional function of reinforcing containers. However, the labels obtained from conventional heat-shrinkable polyester films could not provide such a reinforcing effect sufficiently.
The present invention is completed taking into account the situation above, and an object thereof is to provide a heat-shrinkable polyester film having a potential of exhibiting a favorable label shrinkage property even when used as a full label, and having an additional function of reinforcing the container loaded and shrunk therewith, and a heat-shrinkable label using the same.